gnulib-tool: In sh+py mode, don't fail because of dangling symlinks. Reported by Paul Eggert in <https://lists.gnu.org/archive/html/bug-gnulib/2024-04/msg00318.html> and Pádraig Brady in <https://lists.gnu.org/archive/html/bug-gnulib/2024-04/msg00321.html>. * gnulib-tool: In sh+py mode, use diff option '--no-dereference' if available.